1. Field of the Invention
The present invention relates to the structure of a torque converter used in a power transmission system of a motor vehicle.
2. Description of the Prior Art
As known in the art, a torque converter is principally constructed to have three wheels, namely, an impeller, a turbine and a stator, and its interior is filled with an oil. The impeller that is connected to an input shaft of the torque converter converts rotary force from the engine into flow of the oil, and the turbine that is connected to an output shaft of the converter receives the flow of the oil and converts it into torque. In the meantime, the stator changes the direction of flow of the oil from the turbine, so as to perform a function of increasing the transmitted torque.
Since the oil is a transmitting medium that circulates between the input side and output side of the torque converter, the impeller and turbine connected to the input and output shafts of the converter are allowed to slide relative to each other, to function as a clutch so as to make it easy for the vehicle to be started, and absorb shocks. Due to these advantages as well as the above-described torque increasing function, torque converters are widely used in passenger cars and other types of motor vehicles. On the other hand, there has been a strong demand for reduction of the size of the torque converter, in order to permit the converter to be installed on a front drive vehicle, or incorporate a lockup clutch mechanism, for example. To this end, many torque converters have a reduced axial dimension, and assume a flat shape as seen in its cross section. Thus, the torque converter has been desired to improve its performance while meeting the demand for reduction in the size.
FIG. 13 and FIG. 14 are views schematically showing the flow of an oil in a torque converter. As shown in FIG. 13, a stator 53 serves to change the direction of flow of the oil so as to cause an impeller 51 to be further rotated, thereby to increase the torque as described above. Thus, a large torque ratio t (=torque of output shaft/torque of input shaft) can be obtained where the speed ratio of the output shaft to the input shaft (rotating speed of the output shaft/rotating speed of input shaft) is small.
As the speed ratio approaches 1 where the impeller 51 and turbine 52 are rotated at substantially the same speed, the direction of flow of the oil into the stator 53 is changed, causing a reverse effect. In this case, therefore, the stator is brought into an idling condition by means of a one-way clutch, so as not to reduce the torque ratio. As a result, the oil flows into and out of the stator 53 in substantially the same direction, as shown in FIG. 14. Since the stator does not exert a force to change the flow direction, it does not perform the torque increasing function. This point of operation is called "coupling point".
The performance of the torque converter may be represented by the above-indicated speed ratio e, torque ratio t, transmission efficiency .eta. (=horse power of the output shaft/horse power of the input shaft) and torque capacity T (=required torque for the rotating speed of the input shaft), which are indicated by operating characteristic curves as shown in FIG. 15. When the speed ratio e is in a region smaller than the coupling point, the torque converter performs its torque increasing function, so that the torque ratio exceeds 1, and reaches its maximum of about 2 upon stalling when the output shaft is stopped. With the torque thus increased, the transmission efficiency .eta. reaches its maximum just before the coupling point.
During normal running of the vehicle, in general, the torque converter operates in a region where the speed ratio is larger than that of the coupling point. When a large accelerating force is needed as in the case where the vehicle is being started or accelerated to pass another vehicle, the torque converter operates in a region in which the speed ratio is small, so as to provide the torque increasing function. However, the actual frequency of use of each region is also influenced by the torque capacity.
The torque capacity represents torque that can be received at a certain rotating speed. If this torque capacity is small, the frequency of use of a region having a relatively large torque ratio is increased with a result of improved accelerating performance, but the fuel economy is deteriorated. If the torque capacity is large, on the other hand, the frequency of use of a region having a relatively high transmission efficiency is increased, but the accelerating performance is deteriorated.
When the torque converter has a flat shape in cross section, to be thus small-sized as described above, however, flow separation tends to occur at portions where the flow of the oil is suddenly changed in direction, as indicated by x, y in FIG. 16, which also results in reduction in the effective flow path area and reduced torque capacity.